1. Field of the Invention
The present invention relates to optical network systems, and more particularly, to an optical network system enabling a plurality of nodes to send/receive optical signals to/from one another.
2. Description of the Related Art
As a result of the recent advance in information communication networks, there has been an increasing demand for simultaneous logical or physical connections of numerous nodes on a network. In the case of a communication system called metro access network interconnecting nodes located in relatively near cities, for example, there has been a demand for schemes that logically combine individual nodes on the network together in the form of a mesh to allow each node to communicate with the other nodes.
A network enabling simultaneous communications of multiple nodes is called full mesh network and has hitherto been drawing attention in the fields of inter-office networking and grid computing as a low-cost, small-scale network permitting unbound communication between nodes.
In optical networks, on the other hand, WDM (Wavelength Division Multiplex) scheme has been advancing whereby multiple optical signals of different wavelengths are multiplexed and simultaneously transmitted over a single optical fiber. Also, for individual nodes on optical networks, increasing importance has been placed on the function as an OADM (Optical Add/Drop Multiplexer) for dropping/adding optical signals of specific wavelengths from/to a wavelength division multiplexed signal beam.
A technique to be applied to conventional optical networks has been proposed wherein a dielectric multilayer film is used as a filter for the splitter or coupler of a node, and fiber Bragg grating is employed in a split light leak shutoff section between the splitter and the coupler (e.g., Unexamined Japanese Patent Publication No. 2002-214473 (paragraph nos. [0014] to [0017], FIG. 1)).
In the case of directly routing optical signals by means of the OADM function without converting the optical signals to electrical signals, the optical signal output from a node is routed to propagate through multiple paths. In such cases, if identical optical signals propagated through different paths reach a certain node at the same time, then the node receives exactly the same optical signal in duplicate via different paths, which makes it impossible to carry out normal optical transmission.
FIG. 15 illustrates such duplicate reception of an optical signal. Nodes 101 to 104 are connected in the form of a mesh as shown in the figure. The node 101 generates an optical signal with a wavelength λa carrying certain information (information “a”) (the optical signal with the wavelength λa is added to a wavelength division multiplexed (WDM) signal by the OADM). The signal multiplexed with the wavelength λa is routed directly without being converted to an electrical signal at individual nodes. Let us suppose that the wavelength division multiplexed signal is propagated through paths L1 and L2.
In this case, the node 103 receives, at the same time, the wavelength division multiplexed signal propagated through the path L1 and that propagated through the path L2, and these signals both contain the optical signal with the wavelength λa carrying the information “a” generated by the node 101, which means that exactly the same optical signal is received in duplicate via different paths. Consequently, interference occurs at the node 103, disrupting the communication.
When directly routing optical signals across an optical network, therefore, condition must be fulfilled that an optical signal generated by each node should be transmitted to all other nodes but there should not exist a node that receives, at the same time, identical optical signals propagated through different paths.
In the case of a simple network topology such as a one-dimensional ring network in which multiple nodes are connected in the form of a ring, a WDM optical signal is propagated in one direction, clockwise or counterclockwise. Thus, since a situation where a node receives the same signal in duplicate via different paths does not occur, optical signals can be directly routed. However, in the case of a more complex network topology, it is difficult to perform routing while satisfying the above condition.
Let us consider an exemplary case where an urban area mesh network is configured by an optical network using optical fibers and optical signals are directly routed across the optical network. In the mesh topology, however, each node receives optical signals from all directions, and accordingly, it is difficult to fulfill the condition that none of the nodes should receive the same optical signal in duplicate. A problem therefore arises in that complex network topology cannot be implemented in optical networks in which optical signals are directly routed.
In the case of an existing optical network with relatively complex topology, it is not possible to directly route optical signals across such a network while at the same time satisfying the condition that an optical signal should reach all nodes but no node should receive an identical optical signal in duplicate. In this case, it is necessary that each node perform electrical routing control. That is, optical signals need to be converted to electrical signals to allow destination search and the like to be conducted on the basis of the information indicated by the electrical signals, and then the electrical signals need to be again converted to optical signals, giving rise to a problem that restrictions are placed on the transmission characteristics that optical signals inherently have.